The attack happened towards the end of a midday service in the Metropolitan Cathedral in Campinas city, police say.
The gunman, carrying a revolver and a pistol, rose from his seat and started shooting randomly, Brazil's O Globo News reports.
He then killed himself in front of the altar before police could arrest him.
The motive for the attack is not yet clear. Neither the gunman nor his victims have been identified.
Major Adriano Augusto told O Globo News that the gunman stood up shortly after the prayers had ended.
